Dae-Jin Yang

Dye-sensitized solar cells using network structure of electrospun ZnO nanofiber mats

Nanostructured semiconducting metal oxides and particularly nanofiber-based photoelectrodes can provide enhanced energy conversion efficiencies in dye-sensitized solar cells (DSSCs). In this study ZnO/poly(vinyl acetate) composite nanofiber mats were directly electrospun onto a glass substrate coated with F:SnO2, then hot pressed at 120°C and calcined at 450°C. This resulted in multiple nanofiber networks composed of a twisted structure of 200–500nm diameter cores with ∼30nm single grains. The DSSCs using ZnO nanofiber mats exhibited a conversion efficiency of 1.34% under 100mW∕cm2 (AM-1.5G) illumination.

Self-Assembled Single-Phase Perovskite Nanocomposite Thin Films

Thin films of perovskite-structured oxides with general formula ABO(3) have great potential in electronic devices because of their unique properties, which include the high dielectric constant of titanates, (1) high-T(C) superconductivity in cuprates, (2) and colossal magnetoresistance in manganites. (3) These properties are intimately dependent on, and can therefore be tailored by, the microstructure, orientation, and strain state of the film. Here, we demonstrate the growth of cubic Sr(Ti,Fe)O(3) (STF) films with an unusual self-assembled nanocomposite microstructure consisting of (100) and (110)-oriented crystals, both of which grow epitaxially with respect to the Si substrate and which are therefore homoepitaxial with each other. These structures differ from previously reported self-assembled oxide nanocomposites, which consist either of two different materials (4-7) or of single-phase distorted-cubic materials that exhibit two or more variants. (8-12) Moreover, an epitaxial nanocomposite SrTiO(3) overlayer can be grown on the STF, extending the range of compositions over which this microstructure can be formed. This offers the potential for the implementation of self-organized optical/ferromagnetic or ferromagnetic/ferroelectric hybrid nanostructures integrated on technologically important Si substrates with applications in magnetooptical or spintronic devices.

Vertically Oriented Titania Nanotubes Prepared by Anodic Oxidation on Si Substrates

Vertically oriented titania nanotube arrays were fabricated by anodization of titanium film deposited on silicon substrates under different processing conditions. The anodic formation of nanoporous titania on silicon substrate was investigated in aqueous solutions mixed with highly corrosive Na2SO4/NaF/citric acid. In the result of the anodization of titanium film deposited at room temperature, a very thin layer of ~70 nm having a worm-like structure was grown on the top of the porous layer. But, in the case of titanium film deposited at 500deg, vertically oriented TiO2 nanotube arrays were formed. The average tube outer diameter of the nanotube was 74 nm to 100 nm. The longest nanotube of 681 mum was obtained at 15 V and 30 min. The current density transient curve recorded during anodization under a constant voltage showed a typical behavior for self-organized pore formation.

Vertically oriented TiO 2 nanotube arrays prepared by anodic oxidation

Vertically oriented TiO2 nanotube arrays transformed from Ti thin film on silicon substrates are very attractive due to their potential for nanostructure integration on silicon [1]. We fabricated the vertically oriented TiO2 nanotube arrays using Ti thin film anodization under various processing conditions. To obtain the optimum condition for the vertically oriented TiO2 nanotube arrays, the various anodization voltages and temperatures were investigated in aqueous solutions mixed with highly corrosive Na2SO4/NaF/citric acid and NH4F in glycerol (1,2,3-propanetriol). In case of a good quality Ti thin film deposited at 500degC, vertically oriented TiO2 nanotube arrays were formed in NH4F in glycerol (1,2,3-propanetriol) at 20degC. FE-SEM images of vertically oriented and highly ordered TiO2 nanotube arrays were shown in Fig. 1. The pore diameter and center to center spacing of nanotube arrays were -50 nm and -100 nm, respectively. However, in case of Ti thin film deposited at room temperature, very thin TiO2 layer of-70 nm with a worm-like structure was grown on the porous layer [2]. Fig. 2 shows a typical current density transient curve recorded during anodization under a constant voltage for self-organized pore formation. The pore formation behavior of anodic titanium oxide was similar to that of porous anodic alumina (PAA) [3]. And the dependences of pore morphology and pore formation rate on process parameters were evaluated in Fig 3. The porous anodic oxide layer grown on titanium by electrochemical anodization was studied and compared to the mechanism governing the formation of porous alumina. X-ray diffraction patterns revealed that as-anodized titania nanotube arrays with amorphous phase were crystallized to anatase phase by annealing at 500degC. The fabrication of nanoporous anatase titania with well-defined cylindrical pores and adjustable pore spacing has been attracting considerable fundamental and technological interests because of the important applications in sensing and detection, separation, electronics, optoelectronics and as host materials for various nanostructures [4, 5]. The vertically oriented and highly ordered TiO2 nanotube arrays have applications to dye-sensitized solar cell and bio-sensing materials.

High tunability (Ba,Sr)TiO/sub 3/ thin films on atomic layer deposited buffer layers for Si integration

In this study, we report on Si integration of Ba/sub 0.6/Sr/sub 0.4/TiO/sub 3/ (BST) thin film based microwave tunable devices by use of TiO/sub 2/ films as microwave buffer layer between BST and Si substrates. TiO/sub 2/ buffer layer were grown by atomic layer deposition (ALD) onto Si substrates followed by pulsed laser deposition (PLD) of BST thin films onto the TiO/sub 2/ buffer layer. The interdigital capacitor (IDC) fabricated on BST films grown on TiO/sub 2//high resistivity Si substrates showed much enhanced tunability value of 33.2% while retaining an appropriate Q factor, as compared to 21% value obtained with BST films grown on MgO single crystal substrates. The microwave phase shifters were fabricated on BST thin films to investigate the potential feasibility of integrating BST films as microwave tunable devices. The phase shifter fabricated on BST films grown on TiO/sub 2//Si substrate showed better figure of merit (FOM) of 30.7/spl deg//dB, as compared to 12.1/spl deg//dB of BST/MgO structure. ALD grown TiO/sub 2/ buffer layers enable successful integration of BST based microwave tunable devices onto high resistivity Si wafers.